Transplantation is the preferred therapy for chronic and sometimes for acute failure of the kidney, heart, liver and lungs. The clinical application of transplantation is limited not so much by immune mediated rejection as it is by the number of donors. Thus, fewer than half of those who need organ transplants actually received them and those who do receive transplants must wait for months and sometimes years to receive an organ. This problem could be overcome if animal organs, the pig being generally preferred, were used in lieu of human organs for transplantation. However, clinical xenotransplantation, as such, is prevented by very rapid and severe rejection reactions that destroy the graft over a period of hours to days. Recent studies have revealed that the most severe types of xenografts rejection are initiated by two factors. One is the reaction of natural antibodies of the recipient with antigens in the donor organ, activating the complement system. The other factor is the incompatibility of complement regulatory proteins of the donor organ with the complement system of the recipient, rendering the xenogeneic organ highly susceptible in inadvertent injury by complement. The second problem has been overcome by the development of transgenic pigs, expressing human complement regulatory proteins. The problem posed by xenoreactive antibodies and the antigens they recognize is the subject of this application. The overall objectives of the proposed research are first to identify the actual porcine endothelial cell antigens recognized by human xenoreactive natural antibodies and the antibodies elicited by exposure to a xenogeneic organ and determine the rules governing antibody-antigen interaction. Based on strong evidence that the major epitope recognized by natural antibodies is a carbohydrate, Gala1-3Gal, several strategies are proposed which would eliminate the expression of that carbohydrate from a donor organ. Second, the physiologic consequences of the interaction between xenoreactive antibodies and their corresponding targets on endothelial cells will be elucidated. These studies will focus on the coagulant properties of endothelium and the changes in structure and function induced by complement. Therapeutic approaches to dealing with aberrant physiology will be investigated.